Fluorinated surfactants are widely used in fluoropolymer manufacture for the stabilization of polymer dispersion in emulsion polymerization process.
Traditionally, perfluorocarboxylic acid derivatives have been commercially employed in said processes; due to environmental concerns related to the use of these materials, fluorosurfactants comprising oxygen atoms in side chain have attracted increasing attention for this application. Among them, perfluorooxycarboxylates of general formula: RfO—CF2CF2—O—CF2—COOX, with Rf being a perfluoro(oxy)radical have been considered.
Among synthetic methods for manufacturing materials as above detailed, mention can be made of oligomerization of tetrafluoroethylene epoxide as described in U.S. Pat. No. 3,271,341 (E.I. DU PONT DE NEMOURS) 6 Sep. 1966. According to this method, tetrafluoroethylene epoxide is polymerized in the presence of a suitable free-radical forming compound (e.g. activated charcoal) at low temperature, resulting in polyethers having general formula: CF3CF2—O—(CF2CF2—O)n—CF2COF, with n being an integer from 0 to 10. Subsequent distillation affords various fractions differing in polymerization degree: corresponding acids or salts are obtained from acyl fluoride derivatives by hydrolysis and, for salts, simultaneous or subsequent reaction with a base. Nevertheless, this process suffers from the disadvantage that a distribution of polyether surfactants is obtained, so that yields towards a single particular target compound might be low and separation steps for isolating the same very burdensome.
JP 2006321797 (ASAHI GLASS CO LTD) 30 Nov. 2006 discloses a process for the manufacture of perfluorocarboxylates of formula RF—(O—CF2CF2)k-1—CF2—COOH, with RF being a perfluoroalkyl group, and k being an integer ≧1, by a multi-step process comprising:
(i) esterification of a hydrogenated compound of formula: R—(O—CH2CH2)k—OH, with R being a C1-10 hydrocarbon group with a fluorinated (poly)acyl fluoride of formula Q(COF)n (Q being a n-valent hydrocarbon group; n≧1) yielding corresponding ester of formula: Q[C(O)—O—(CH2CH2O)k—R]n;(ii) complete fluorination of all C—H bonds in C—F bonds of this latter to yield corresponding perfluorinated compound of formula QF[C(O)—O—(CF2CF2O)k—RF]n;(iii) decomposition of the perfluoroester to yield perfluoroacyl compound of formula F—C(O)—CF2O—(CF2CF2O)k-1—RF;(iv) hydrolysis and/or treatment with a base for yielding corresponding carboxylate salt.
Major drawback of this process is its significant consumption of fluorine in fluorination step, as raw material providing backbone of the final perfluorocarboxylates is initially provided as fully hydrogenated precursor, so that severe fluorination conditions are required for assuring quantitative fluorination towards the target compound.
Scientific paper from WROBLEWSKA, A., et al. Synthesis of technicall useful perfluorocarboxylic acids. Journal of Fluorine Chemistry. 2006, vol. 127, p. 345-350. discloses a process for manufacturing perfluorocarboxylic acids via dehydroiodination of perfluoroalkylethene iodides and subsequent oxidation of ethylenically unsaturated intermediate therefrom.
WO 2007/014009 (3M INNOVATIVE PROPERTIES COMPANY) 6 Dec. 2007 discloses surfactants of formula [RfO-L-COO−]iXi+, wherein L is a linear partially or fully fluorinated alkylene group or aliphatic hydrocarbon group and Rf is a linear partially or fully fluorinated aliphatic group interrupted with one or more oxygen atoms, Xi+ represents a cation having the valence i=1, 2 or 3. Certain surfactants encompassed by above mentioned formula can be prepared by reacting certain fluorinated olefins with a hydrocarbon alcohol in an alkaline medium, and then decomposing the resulting ether in acidic conditions thereby forming the corresponding carboxylic acid. Among suitable alcohols, mention is made of methanol, ethanol, butanol. Nevertheless, this process does not enable manufacture of fully fluorinated carboxylates.
It was thus felt a need in the art for providing a novel process for manufacturing perfluorooxycarboxylates of general formula: RfO—CF2CF2—O—CF2—COOX, which could override drawbacks of the prior art.